Each day more than 1,500 children become infected with human immunodeficiency virus type 1 (HIV-1) worldwide, most in developing countries where nearly 40% of infant infections result from extended breastfeeding. Following ingestion of HIV-infected breastmilk, gut mucosal surfaces are the most likely site where virus transmission occurs. One of the most profound changes affecting the gut mucosa of newborns is the acquisition at birth of a diverse population of commensal bacteria. Among these, lactic acid bacteria (LAB) are a normal constituent of the microbiota of the gastrointestinal tract, where they help strengthen the mucosal barrier and modulate immune reactivity against foreign antigens and enteric pathogens. Despite considerable evidence that LAB play an important role in enhancing mucosal defenses, it is not known whether they can inhibit post-partum HIV-1 infection acquired through breast-feeding. In this proposal, we will evaluate the hypothesis that LAB contribute directly to the intestinal mucosal defense against HIV-1 through secretion of novel virus inhibitory factors that interfere with CCR5. HIV-1 is selectively transferred across intestinal epithelial cells by transport mechanisms involving CCR5 and we hypothesize that LAB may interfere with this process. To date, no studies have been conducted in neonates to identify intestinal strains of LAB with anti-HIV activity and few data exist on the mechanism(s) of LAB inhibition of HIV infection across mucosal surfaces. The specific aims of this proposal are two-fold: 1) to identify LAB with anti-HIV activity in neonatal stool samples in support of their potential role in muscoal defense, and 2) to investigate the mechanism of HIV-1 inhibitory actjvity in vitro using a diverse panel of primary HIV-1 isolates and intestinal epithelial cell transport models to determine whether inhibition involves CCR5. The long-term goal of these studies is to identify LAB with anti-HIV activity that can be utilized in developing countries as a means of augmenting the natural mucosal defense barrier in both lactating mothers and their breast-fed infants.